Fix Vite proxy inside Docker and add success pages

- vite.config.ts: proxy target via VITE_API_TARGET env var (falls back to localhost)
- docker-compose.dev.yml: set VITE_API_TARGET=http://backend:8000
- Add /login-success and /register-success placeholder pages
- Show real API error messages in login/register forms

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -6,7 +6,7 @@ export default defineConfig({
   server: {
     proxy: {
       "/api": {
-        target: "http://localhost:8000",
+        target: process.env.VITE_API_TARGET ?? "http://localhost:8000",
         changeOrigin: true,
       },
     },